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//compile with: gcc -Wall -pthread sharedvariable_posix.c -o svar
//using POSIX named semaphores

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <fcntl.h>
#include <sys/shm.h>
#include <semaphore.h>


int * shared_var;
int shmid;
sem_t *mutex;

void worker()
	{
	usleep(1000000 + rand()%11*100000); //uses microseconds sleep and waits 1 to 2 seconds in periods of 0.1 secs
        sem_wait(mutex);
        (*shared_var)++;
        sem_post(mutex);
	}

int main(int argc, char *argv[])
	{
	int i, n_procs;

	if(argc!=2) {
		printf("Wrong number of parameters\n");
		exit(0);
		}

	n_procs=atoi(argv[1]);

	// Create shared memory
	if( (shmid = shmget(IPC_PRIVATE,sizeof(int),IPC_CREAT|0766)) > 0) {
                printf("\nShared memory created\n");
        }

        else {
                perror("\nERROR IN CREATING MEMORY\n");
                exit(1);
        }

        // Attach shared memory
        shared_var = (int *) shmat(shmid,NULL,0);

        // Create semaphores
	sem_unlink("MUTEX");
        mutex = sem_open("MUTEX",O_CREAT|O_EXCL,0700,1);

        // initialize variable in shared memory
        (*shared_var) = 1000;

        // Create worker processes
        for(i = 0; i < n_procs; i++) {
                if ( fork() == 0 ) {
                        worker();
                exit(0);
                }
        }
        // Wait for all worker processes

        for(i = 0; i < n_procs; i++) wait(NULL);

        // print final result
        printf("Shared Var: %d\n",(*shared_var));

        // remove resources (shared memory and semaphores)
        sem_close(mutex);
	sem_unlink("MUTEX");
	shmdt(shared_var);
        shmctl(shmid,IPC_RMID,NULL);

	exit(0);
}